The motivations


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OBELIX is an experiment designed to study exclusive final states of antiproton and antineutron annihilations at low energies, with protons and nuclei.

The physics motivations are:

Meson spectroscopy

Meson spectroscopy is not an end in itself, it must be rather considered a tool for the study of Quantum Chromo Dynamics (QCD) in the non-perturbative sector.
The mathematical techniques of perturbative theory, so successful in QED, can be applied with success to short distance phenomena (very high energy), where QCD and QED are similar in manifestations. But we do not yet know how to describe the long range confinement properties from first principles. This is a frustrating aspect of QCD.
The depth of our ignorance can be illustrated by the fact that although it is widely accepted that hadrons are either baryons (3q objects) or meson (q qbar objects), we cannot explain this using QCD.
Moreover, an unvindicated expectation from QCD, is the existence of a new form of matter, the glueballs (gg or ggg) and the hybrids (q g qbar).

A rather established consensus in the theoretical/phenomenological activity concerns the prediction of the mass region which should be populated by gluonium and hybrids states.
These mesonic resonances are expected in the low mass region of the hadron spectrum (between 1.0 and 2.5 GeV/c^2). Since this region is overpopulate by the standard q qbar mesons and their radial excitations, and a non qbar q object may have allowed qbar q quantum numbers, the task of identifying exotic states is not simple.
The strategy developed in the last few years is to use reactions and processes which can tag the flavor content or the quantum numbers of the produced states and then to compare the production properties of a given new state in different reactions.

OBELIX has focussed its search on the mass region (below 1.8 GeV/c^2), with the am of comparing different type of reactions, specifically with final states containing kaons, taking also advantage of the possibility to change the distribution of the protonium initial states by varying the target densitiy, and, with the nbar beam, to have a pure isospin I=1 in the initial state.

The dynamics of the N Nbar interactions

Another important field of research of OBELIX is the study of the annihilation dynamics through the measurement of two-body annihilation frequencies at rest and of the annihilation cross sections in flight at low energy.
The possibility to extract from these data the hadronic branching ratios offers the possibilityt of a direct comparison with the theoratical predictions on quark dynamics.

The exploration of the validity of the OZI rule has suggested to OBELIX a systematic study of antinucleon annihilations in the channels omega pi+- and omega pi+-, using both antiproto and antineutron beams. The dramatic OZI violation experimentally observed and the development of the analyses in progress will allows to check the correctness of the hypothesis of the intrinsic strangeness content of the nucleon.

Nuclear and Atomic physics

The Collaboration performed also some measurements in fields different from meson spectroscopy and annihilation dynamics. The topics chosen have the common characteristics to be of primary importance in the respective areas of nuclear and atomic physics and to require a very short time for data taking.